Differential pressure sensor

ABSTRACT

There is disclosed a differential pressure sensor with a first and a second measuring chamber. Each measuring chamber being limited by a rigid carrier plate and a diaphragm plate, which is formed in the region of the measuring chamber as a pressure-sensitive measuring diaphragm. To design the differential pressure sensor to be resistant to overloading, the carrier plate is arranged between a first and a second diaphragm plate and has congruent concave depressions on opposite sides in the plane of the plate. The depressions are connected to one another by a central duct, penetrating the carrier plate perpendicularly to the plane of the plates. In the region of the measuring chambers, the diaphragm plates are formed congruently in relation to the depressions as pressure-sensitive measuring diaphragms. The measuring chambers are coupled to one another by a ram guided axially movably in the duct.

FIELD OF THE INVENTION

[0001] This invention relates to a differential pressure sensor madeusing glass-silicon technology and more particularly to such a sensorthat is useful for process measurements.

DESCRIPTION OF THE PRIOR ART

[0002] DE 42 07 949 discloses a capacitive differential pressure sensormade using glass-silicon technology in which a plate of silicon, servingas a pressure-sensitive diaphragm and as a first electrode, is arrangedbetween two carrier plates consisting of glass, the silicon plate andthe carrier plate being integrally connected to one another in theiredge region by anodic bonding in such a way that in each case a carrierplate combines with the silicon plate serving as the diaphragm to form ameasuring chamber, each carrier plate has a pressure supply duct, whichruns perpendicular to the contact surfaces of the silicon plate and ofthe carrier plates and via which the respective measuring chamber can bepressurized, and the surfaces of the carrier plates lying opposite thedeflectable region of the silicon plate serving as the diaphragm areeach provided with a metallization, serving as a second electrode, insuch a way that the first electrode and the second electrodes form adifferential-pressure-dependent capacitor arrangement.

[0003] The differential-pressure-dependent deformation of the plateserving as a diaphragm brings about a change in capacitance of thecapacitor arrangement, the change in capacitance being a direct measureof the differential pressure. The change in capacitance is measuredelectrically. The capacitor arrangement is connected to a measured-valueprocessing device via connecting conductors.

[0004] In addition, German Utility Model DE 200 19 067 discloses apressure-measuring device with a piezoresistive pressure sensor andhydraulic force transmission in which the process pressure of themeasuring medium is transmitted to the pressure sensor by interposing aseparating diaphragm with a fluid diaphragm seal, theprocess-pressure-dependent, diaphragm-seal-displacing deflection of theseparating diaphragm being mechanically limited to an amountprescribably exceeding the measuring range, and the pressure sensorbeing arranged in the pressure-measuring device in such a way that itcan move on a mechanically pretensioned overload diaphragm which, independence on process pressure exceeding the measuring range, limits avolumetrically variable equalizing space for accepting excess diaphragmseal.

[0005] In both cases, the measuring principle is based on thedeformation of a diaphragm by the differential pressure present on bothsides of the diaphragm. The rigidity of the measuring diaphragms ischosen on the one hand such that as great a deflection as possible isproduced in the differential pressure range to be detected, andconsequently the greatest possible excursion of the output signal isproduced. On the other hand, the rigidity of the diaphragm must be sogreat that, in the case of overloading at differential pressures abovethe measuring range, destruction of or damage to the diaphragm isavoided.

[0006] A typical value for the required overload resistance ofsilicon-diaphragm differential pressure sensors is four times thedifferential pressure of the measuring range end value. This is adequatefor many applications, in particular for atmospheric pressuremeasurement. By contrast, in process measuring technology there are manyknown applications in which, for example, a measuring range end value of1 kPa is required in combination with an overload resistance of 40 Mpa.Such overload resistances are achieved in conformity with DE 200 19 067by what is known as a Florentine flask and an arrangement of additionaldiaphragms, which limit the maximum differential pressure at the sensorcell to a permissible value.

[0007] The interconnected separating diaphragms with a fluid pressureseal disadvantageously represent a considerable cost factor in thefabrication of the pressure-measuring device, amounting to many timesthe cost of the differential pressure sensor.

[0008] In addition, the properties of the separating diaphragmsadversely influence the sensor properties, in particular in the case ofdifferential pressure sensors for low differential pressures. Therigidity of the separating diaphragms reduces the measurement dynamicsand the responsiveness at the beginning of the measuring range.

[0009] The construction with external separating diaphragms hindersminiaturization of the pressure-measuring device and consequently use inapplications where space is critical.

[0010] The invention is therefore based on the object of specifying anoverload-resistant differential pressure sensor which manages withoutexternal separating diaphragms for its protection. The present inventionachieves that object.

SUMMARY OF THE INVENTION

[0011] The present invention proceeds from a differential pressuresensor with a first and a second measuring chamber, each measuringchamber being limited by a rigid carrier plate and a diaphragm plate,which is formed in the region of the measuring chamber as apressure-sensitive measuring diaphragm.

[0012] According to the invention, a single carrier plate is arrangedbetween a first and a second diaphragm plate. The carrier plate hascongruent concave depressions on opposite sides in the plane of theplate. The depressions are connected to one another by a central duct,penetrating the carrier plate perpendicularly to the plane of theplates. In the region of the measuring chambers, the diaphragm platesare formed congruently in relation to the depressions aspressure-sensitive measuring diaphragms, each measuring chamber beingformed by the space between the surface in each case of a concavedepression and the surface facing the carrier plate of the associatedmeasuring diaphragm. The measuring diaphragms are coupled to one anotherby a ram guided axially movably in the duct.

[0013] The concave depressions and the rigidity of the measuringdiaphragms are dimensioned in this case in such a way that the measuringdiaphragms are freely movable in the measuring range of the differentialpressure sensor.

[0014] The sides of the measuring diaphragms facing away from thecarrier plate are subjected to the process pressures. In this case, thefirst measuring diaphragm is loaded with the first process pressure andthe second measuring diaphragm is loaded with the second processpressure.

[0015] If the two measuring diaphragms are subjected to pressureasymmetrically, the measuring diaphragm which is subjected to thestronger pressure curves convexly in the direction of the carrier plate,into the space of the adjoining measuring chamber. The ram causes theother measuring diaphragm to curve convexly away from the carrier plateby the same amount.

[0016] If they are asymmetrically subjected to pressure exceeding themeasuring range, the measuring diaphragm which is subjected to thestronger pressure comes to bear against the surface of the concavedepression. The deflection of the measuring diaphragm which is subjectedto the smaller pressure is limited to the same amount by the ram. As aresult, damage to the measuring diaphragms during overloading isadvantageously avoided. In this case, the differential pressure sensormanages without a separate overload protection system comprisingseparating diaphragms and a Florentine flask.

[0017] In addition, it is advantageously possible to dispense with theinternal oil filling. This makes the production of the differentialpressure sensor according to the invention simpler, and consequentlyless expensive.

[0018] The measuring diaphragms are in direct contact with the processmedium, their mobility in the direction of the process medium not beingrestricted in the measuring range. This prevents instances of damagecaused by jamming of particles entrained in the process medium.

[0019] The slightly curved surface topography of the concave depressionslimits the diaphragm loading in the case of overloading.

[0020] Mechanical coupling of two measuring diaphragms with support onone side in each case achieves the effect of overload resistance on bothsides of the differential pressure sensor.

[0021] The freely accessible, external measuring diaphragms make iteasier to apply anticorrosion coatings to prolong the lifetime of thedifferential pressure sensor in aggressive media.

DESCRIPTION OF THE DRAWING

[0022]FIG. 1 shows a sectional representation through a differentialpressure sensor embodied in accordance with the present invention.

[0023]FIG. 2 shows a representation of a plan view of the carrier plateof the sensor of FIG. 1.

DESCRIPTION OF THE PREFERRED EMBODIMENT(S)

[0024] In FIG. 1, the construction of a differential pressure sensor inaccordance with the present invention is represented in cross section.Using the same reference numerals for the same means, a plan view isshown in FIG. 2.

[0025] The differential pressure sensor comprises a first diaphragmplate 1 and a second diaphragm plate 2, which are connected to a carrierplate 3, in each case in surface-area contact at a gastight joint 5.Both diaphragm plates 1 and 2 have regions of reduced materialthickness, which respectively form a measuring diaphragm 7.

[0026] On each side facing the diaphragm plates 1 and 2, the carrierplate 3 respectively has a concave depression 6. The two depressions 6and also the measuring diaphragms 7 of the diaphragm plates 1 and 2 arecongruent to one another.

[0027] The space between the surface in each case of a concavedepression 6 and of a measuring diaphragm 7 of the diaphragm plates 1and 2 respectively forms a measuring chamber 8. This arrangement allowsthe measuring diaphragm 7 which is subjected to the stronger pressurewhen excessive pressure is present to deflect to such an extent that itbears with surface-area contact against the carrier plate 3. In the caseof overloading, further deflection is prevented by the carrier plate 3.

[0028] The carrier plate 3 additionally has a duct 9, which connects thetwo concave depressions 6 and is arranged away from the depressions 6.In this duct 9 there is a ram 4, which is firmly connected to bothmeasuring diaphragms 7. This coupling has the effect that, in the caseof overloading of the measuring diaphragm 7 which is respectivelysubjected to the stronger pressure, the measuring diaphragm 7 which isrespectively subjected to the weaker pressure is also deflected onlywithin the permissible limits.

[0029] The carrier plate 3 preferably consists of glass, which isconnected to the diaphragm plates 1 and 2 at the joints 5 by anodicbonding. The diaphragm plates 1 and 2 consist of silicon. The measuringdiaphragms 7 are structured by etching processes.

[0030] In a first refinement of the invention, it may be envisaged toread out the differential pressure sensor capacitively to obtain anelectrical measuring signal. For this purpose, mutually isolatedconductive regions are provided as capacitor coatings on the surface ofthe depressions 6 and each side of the measuring diaphragm 7 facing adepression 6. These conductive regions preferably consist of gold. Forthe capacitor coatings on the measuring diaphragms 7 consisting ofsilicon, a local doping of the silicon may alternatively be provided.These conductive regions are connected to an evaluation circuit. Theelectrical connections between the conductive regions and the evaluationcircuit are arranged on the glass surface. Alternatively, the electricalconnections may be arranged on the silicon surface.

[0031] In an alternative refinement of the invention, it may beenvisaged to read out the differential pressure sensor piezoresistively.For this purpose, mutually isolated piezoresistive conductor tracks,which are preferably formed by doped silicon, are arranged on themeasuring diaphragms 7. A high sensitivity of the differential pressuresensor is achieved by four piezo resistors in a bridge circuit.

[0032] The ram 4, like the carrier plate 3, consists of glass and isconnected to the measuring diaphragms 7 by aniodic bonding. In analternative refinement, it may be provided that the carrier plate 3consists of silicon. It may additionally be provided that the carrierplate 3 is connected to the measuring diaphragms 7 by fusion bonding.Alternatively, the carrier plate 3 may be joined to the measuringdiaphragms 7 by eutectic bonding.

[0033] In a further refinement of the invention, it is envisaged tocover the surfaces of the diaphragm plates 1 and 2 facing away from thecarrier plate 3 with a protective layer for protection against damagecaused by aggressive process media. In a first embodiment, theprotective layer consists of diamond. In an alternative embodiment, aprotective layer of silicon nitride is provided, which can be producedin a particularly advantageous way by processes known per se for thesurface nitriding of silicon. In a third embodiment, a coating with acorrosion-resistant metal is provided.

[0034] It is to be understood that the description of the preferredembodiment(s) is (are) intended to be only illustrative, rather thanexhaustive, of the present invention. Those of ordinary skill will beable to make certain additions, deletions, and/or modifications to theembodiment(s) of the disclosed subject matter without departing from thespirit of the invention or its scope, as defined by the appended claims.

What is claimed is:
 1. A differential pressure sensor comprising: arigid carrier plate arranged between first and second diaphragm plates,said carrier plate having congruent concave depressions on oppositesides of said carrier plate; a central duct penetrating said carrierplate perpendicularly to the plane of said first and second diaphragmplates, said duct connecting said depressions to each other; first andsecond measuring chambers, each of said chambers limited by said carrierplate and an associated one of said first and said second diaphragmplates; said first and said second diaphragm plates in the region ofsaid associated one of said first and said second measuring chambers areeach formed congruently as pressure sensitive measuring diaphragms inrelation to an associated one of concave depressions in said carrierplate; and a ram guided axially movable in said duct coupling saidpressure sensitive measuring diaphragms to each other.
 2. Thedifferential pressure sensor of claim 1 wherein said carrier plateconsists of glass.
 3. The differential pressure sensor of claim 1wherein said first and second diaphragm plates consist of silicon. 4.The differential pressure sensor of claim 1 wherein said first andsecond diaphragm plates are anodically bonded to said carrier plate. 5.The differential pressure sensor of claim 1 wherein said ram is consistsof glass.
 6. The differential pressure sensor of claim 3 wherein saidram is anodically bonded to said first and second diaphragm plates. 7.The differential pressure sensor of claim 5 wherein said ram isanodically bonded to said first and second diaphragm plates.
 8. Thedifferential pressure sensor of claim 1 wherein the surfaces of saidfirst and second diaphragm plates facing away from said carrier plateare coated with diamond.
 9. The differential pressure sensor of claim 1wherein the surfaces of said first and second diaphragm plates facingaway from the carrier plate are coated with silicon nitride.
 10. Thedifferential pressure sensor of claim 1 wherein the surfaces of saidfirst and second diaphragm plates facing away from the carrier plate arecoated with a corrosion-resistant metal.
 11. The differential pressuresensor of claim 1 wherein the surfaces of said first and secondmeasuring diaphragms facing said carrier plate and the surfaces of saidconcave depressions are conductively coated in such a way that they areisolated from one another.
 12. The differential pressure sensor of claim11 wherein said conductive coating consists of gold.
 13. Thedifferential pressure sensor of claim 11 wherein said conductive coatingof the surfaces of said first and second measuring diaphragms is formedby a local doping of silicon.
 14. The differential pressure sensor ofclaim 1 wherein mutually isolated piezoresistive conductor tracks arearranged on the surfaces of said first and second measuring diaphragmsfacing said carrier plate.
 15. The differential pressure sensor of claim11 wherein said piezoresistive conductor tracks are formed by a localdoping of silicon.
 16. A method for making a differential pressuresensor with first and second measuring chambers comprising: a. providingcongruent concave depressions on opposite sides of a rigid carrier platein the plane of said carrier plate; b. arranging said carrier platebetween first and second diaphragm plates, said first and seconddiaphragm plates formed congruently in the region of an associated oneof said concave depressions as first and second pressure sensitivemeasuring diaphragms; c. connecting said concave depressions to eachother by a central duct which penetrates said carrier plateperpendicularly to the plane of said first and second diaphragm plates;and d. coupling said first and second measuring diaphragms to each otherby a ram guided axially movably in said duct.
 17. The method of claim 16further comprising anodically bonding said first and second diaphragmplates to said carrier plate.
 18. The method of claim 16 wherein saidfirst and second diaphragm plates consist of gas and further comprisinganodically bonding said ram to said first and second diaphragm plates.19. The method of claim 16 wherein said ram consists of glass andfurther comprising anodically bonding said ram to said first and seconddiaphragm plates.
 20. The method of claim 16 further comprising coatingwith diamond the surfaces of said first and second diaphragm platesfacing away from said carrier plate.
 21. The method of claim 16 furthercomprising coating with silicon nitride the surfaces of said first andsecond diaphragm plates facing away from said carrier plate.
 22. Themethod of claim 16 further comprising coating with a corrosion-resistantmetal the surfaces of said first and second diaphragm plates facing awayfrom said carrier plate.
 23. The method of claim 16 further comprisingconductively coating the surfaces of said first and second diaphragmplates facing said carrier plate and the surfaces of said concavedepressions in such a way that they are isolated from one another. 24.The method of claim 16 further comprising arranging mutually isolatedpiezoresistive conductor tracks on the surfaces of said first and secondmeasuring diaphragms facing said carrier plate.
 25. The method of claim24 further comprising forming said piezoresistive conductor tracks by alocal doping of silicon.